The invention relates to an optical apparatus for producing a light beam having a substantially uniform phase front which finds particular utility in electro-optical distance measuring devices.
In recent years, distance measuring instruments have been developed and are available commercially which utilize the measurements and phase of a beam of modulated light to determine the distance between the instrument and some reference object. In such devices, a beam of coherent light, e.g., from an LED, is modulated at some convenient frequency and directed to the object which is to be measured. The phase relation between the transmitted and received light is a measure of the distance from the instrument to the target and can be determined by converting the received signals to electrical signals in a photodiode or the like and using conventional electronic circuits to ascertain and display the phase difference between the respective signals. Such devices are described in greater detail, e.g., in an article by Robin H. Hines, one of the inventors of the present application, entitled, "A Geodic and Survey Infra Red Distance Measurement Instrument", which appeared in SPIE, vol. 95, Modern Utilization of Infra Red Technology II (1976), pp. 204-205.
Since the phase between the received and reflected light is used to calculate the distance, it is obvious that any variation in uniformity in the transmitted phase front will result in inaccuracies in measurement. Accordingly, it is desirable in this application and in other applications that require a precise modulated light phase measurement to minimize the error in the phase front. When a light emitting diode is modulated at a frequency in the hundreds of KHz through the tens of MHz range, or when a continuous-wave laser is electro-optically modulated in a similar frequency range, the output beam does not have a radially uniform phase front. This non-uniformity can probably be attributed to contaminants in the semi-conductor process causing non-uniform transit time within the LED, and due to harmonics and resonances, established in the electro-optical crystal with the laser modulation. Also, the intensity of the light may vary radially in extreme cases and exhibit light hotspots. In any case, the degree of homogeneity of the LED output beam is a function of the manufacturing techniques and materials and is beyond the control of the user.
In addition to the non-uniform phase front, the radiant power emitted by typical LEDs is distributed over large angles with respect to the axis of the device.
Since the effective range of the instrument is a function of the divergence of the generated light beam 5 and the emitted power P.sub.E, ##EQU1## wherein: P.sub.T = Transmitted Power,
.theta. = The Half Angle Collection Cone, and ##EQU2## and for most electro-optical distance measuring devices, the trade-off between the transmit lens diameter and the focal length is such that the half angle is typically from 2 to 6.degree., only a small portion of the energy is within that small half angle.
The present invention relates to a simple and passive optical device which finds particular utility in conjunction with electro-optical measuring devices and other similar apparatus in which it is desirable to produce a uniform phase front. This is accomplished by an optical transmitting tube which has internal reflecting surfaces which diverge, i.e., taper, from each other in the direction from input to output. Light entering the taper, e.g., from an LED, at greater than a predetermined angle, e.g., 4.degree., is internally reflected, bouncing off the internal reflecting surfaces. This internal reflection both mixes the emitted energy to minimize the non-uniform phase front and also concentrates the energy into a narrow beam to permit greater transmitted power. Since the walls are tapered, the original path of each reflected ray is altered to a path more nearly parallel to the axis and, therefore, energy which would not ordinarily be transmitted due to the small half angle is now concentrated within that half angle. Additional concentration can be attained by placing a lens between the optical taper and the LED with the entry end of the tapered tube located at the best focus for the LED.
Other objects and purposes of the invention will be clear from the following detailed description of the drawings.